The present invention is directed to a fiber optic sensing instrument for sensing deflections, displacements, or other physical conditions and more particularly to such a sensing instrument having a fiber of adjustable optical path length, and is further directed to a system and method using such a sensing instrument. The adjustable length can be used for spatial division multiplexing, extending the range of displacements detectable by the sensing instrument, or other purposes.
The use of optical fibers to sense deflections, displacements, temperatures and other physical conditions is well known. Typically, such sensors operate by interferometry. An interferometric fiber optic sensing instrument, in its simplest form, operates by splitting light from a light source between two fibers. The first fiber, a sensing fiber, is exposed to the physical condition to be sensed, while the second fiber, a reference fiber, is not. The light passing through the sensing and reference fibers is recombined; if the difference between the optical path-lengths of the sensing and reference fibers is within the coherence length of the light, an interference fringe dependent on the phase difference between the light passing through the sensing and reference fibers can be detected. The physical condition to be sensed causes the sensing fiber to change its optical path length, e.g., by changing its physical length or its index of refraction. A change in the interference fringe allows the computation of a change in the phase difference, which in turn allows the computation of a change in the optical path length experienced by the sensing fiber, which in turn allows the computation of a quantity of the physical condition.
However, the simplest form has the following drawbacks. First, while it can detect a quantity of the physical condition, it is often relevant where along the sensing fiber the physical condition occurs. For example, if a long fiber is used to sense pressure or temperature over an extended area, the simplest form cannot detect the location of the pressure or temperature in the extended area. Second, the range of phase differences must fall within 2xcfx80; otherwise, the resulting phase ambiguity renders the detection ambiguous or even meaningless.
To overcome the first drawback, various forms of multiplexing are known. For example, U.S. Pat. No. 4,443,700 to Macedo et al teaches an optical sensing apparatus with multiple sensing fibers spaced along its length. Signals from the multiple sensing fibers are distinguished by their time delays. However, it is necessary to resolve such time delays on the order of a few nanoseconds, thus complicating the device and requiring care in selection of the optical fiber such that the pulse dispersion is minimized.
To overcome the second drawback, U.S. Pat. No. 5,721,615 to McBride et al teaches a fiber optic sensing instrument having a sensor arm and a reference arm. The reference arm has a device having a microscope stage for varying a path difference between the sensor and reference arms. Alternatively, one of the arms can be stretched by a clamp. An interferogram is generated when the path lengths are equal. However, fairly complicated mathematics are used to calculate strain and temperature from the group delay and dispersion as determined from the interferogram.
Smartec SA of Manno, Switzerland, advertises a technology for fiber optic interoferometric measurement known as SOFO. Two optical fibers are installed in the pipe or other structure to be monitored; the first is in mechanical contact with the structure to expand or contract therewith and functions as a sensing fiber, while the second is free and functions as a reference fiber. An analyzer for use with such a sensor also has two optical fibers, one of which has a movable mirror to adjust its optical path length. A modulated signal is obtained only when the difference in optical path lengths between the two fibers in the structure is compensated by the difference in optical path lengths between the two fibers in the analyzer to better than the coherence length of the light source. However, the SOFO system introduces an undesirable complexity in that two fibers must be present in the structure to be monitored. Also, the analyzer of the SOFO system cannot demultiplex and analyze signals from multiple sensors without the use of an optical switch to select the signal from one of the sensors.
It will be readily apparent from the above that a need exists in the art for a simple way to overcome the above-noted problems with the prior art. It is therefore a primary object of the invention to provide a fiber optic sensing instrument capable of detecting a wide range of phase differences between the sensing and reference fibers.
It is another object of the invention to provide a fiber optic sensing instrument, system and method capable of detecting a wide range of phase differences so as to detect the location of the physical condition being sensed as well as its quantity.
It is another object of the invention to provide a fiber optic sensing instrument, system and method capable of detecting a wide range of phase differences so as to detect a wide range of displacements or other changes in the optical path length in the sensing fiber.
To achieve the above and other objects, the present invention is directed to a fiber optic sensing system incorporating a fiber having an adjustable optical path length. A sensing optical fiber is applied to a structure to be monitored to detect displacement or the like by changing its optical path length. A reference optical fiber has a fixed optical path length. An adjustable length optical fiber is controllably adjusted in its optical path length. The three optical fibers form optical paths whose light outputs are caused to interfere. The adjustable length optical fiber is adjusted until an interference fringe appears. The quantity to be detected is derived from the maximum of the interference fringe. Several sensing optical fibers can be multiplexed; by staggering their optical path lengths, their interference fringes can be separated sufficiently to resolve them.
One embodiment provides a fiber optic interferometric sensing system having a sensing fiber of any arbitrary length used to measure deflections, or displacements, using a mirrored optical fiber (single mode or multimode). This system can include a single optical fiber bonded to, or attached at discrete points to, or imbedded in a structure, of any shape or configuration. Alternatively, the fiber can be fixed at both ends, with no continuous attachment to a structure.
In addition, many such fiber sensors, acting as individual strain sensors, can be optically coupled to a single backbone fiber, provided each sensor length is different (according to criteria described later) to provide a spatial division multiplexing capability.
These sensors measure displacement, from which an average value of strain can be calculated by dividing the measured displacement by the length of the sensor. These sensors can be of any length, typically ranging from a few centimeters to many meters. The combination of sensor lengths that can be incorporated on the same backbone can also vary from very short lengths (ie, several centimetres) to very long gages (e.g.; up to 100 meters for example).
An optical source of short coherence length (such as a light emitting diode) produces a broadband light beam that is split between the optical fiber sensor, a passive reference optical fiber, and an adjustable length optical fiber which can be actuated by various means to extend or contract its length (assuming an initial pre-tensioned state). Each optical fiber has mirrored ends to reflect the incident light beams. The light from the source thus travels two paths that are recombined at a photodetector.
Upon activating the fiber optic sensor (a single mode fiber is preferred due to losses associated with multimode fibers) by means of structural loading, or any means that leads to extension or contraction of the sensor (such as by temperature changes from the installed reference state, for example), the displacement difference between this sensor and its passive reference sensor is measured by adjusting the adjustable optical fiber until an interference pattern is detected by a photodetector. The peak in the interference pattern occurs when the two optical paths are equal.
The adjustable length optical fiber can be adjusted by any suitable technique, such as a motor drive with the fiber wrapped around cylindrical pulleys for example, or a piezoelectric cylinder having the fiber wrapped around its circumference. The length of this adjustable fiber determines the maximum displacement it can measure, as limited by its tensile breaking strength, i.e., the maximum strain or displacement it can undergo as limited by its strength in tension. The longer the optical fiber, the greater the magnitude of the displacement for a given ultimate strain for the fiber material. For example, the typical maximum displacement for a single mode optical fiber of 3 meters length is 60 mm. The rate at which the adjustable optical fiber can be stretched or contracted, determines the system""s capability to measure dynamic displacement profiles.
The total displacement range of the adjustable optical fiber allows multiple fiber optic sensors of different lengths to be monitored by the same optical light source and passive reference fiber, provided that the sum of the changes in length of all of the sensors is less than the maximum deflection length of the adjustable optical fiber. Spatial (ie; different length fiber optic sensors optically coupled to a single backbone fiber transmitting the light beam from the light source) division multiplexing can be achieved by altering the lengths of the fiber optic sensors in increments, the sum of which is less than or equal to the length of the passive reference optical fiber minus the sum of the predetermined allowable measured deflections associated with the application of each of the sensors coupled to the optical backbone fiber.
Application examples include, but are not limited to, surface bonding the sensors to pipes, pressure vessels, bridge structures of steel or concrete, or imbedding the sensors in concrete or composites. In these embodiments, the sensors can measure displacements in the form of elongation or contraction, which can be converted to strains in tension or compression. It is envisaged that to measure compression or contraction, the sensors are bonded under a pretension load. This will be important for measuring temperature fluctuations for example, which can be below that of the installation temperature, thus leading to possible thermal contraction of the substrate material, depending on its thermal coefficient of expansion.
Other applications that do not require a continuous attachment to a structure include using the pre-tensioned sensors as deflection measuring sensors between two or more fixed points. Spatial division multiplexing can also be used in this configuration.
A very long gage mirrored fiber optic sensor (consisting of a single mode or multimode fiber) capable of measuring average displacements over any gage length, typically varying from about one meter to over a hundred meters, can be implemented. Applications of very long gage fiber optic displacement sensors include bonding them to long pipeline sections to measure changes in the pipe geometry due to such factors as pressure changes, corrosion leading to wall thinning and radial expansion, cracks or leaks leading to gas/fluid loss. Other applications as long gage displacement measuring devices include monitoring the movement of large structures such as dams, due to movement in the earth/concrete foundation over large distances, vibration and creep behaviour of bridges and buildings. These sensors can be used where electrical and semiconductor based strain gages and vibrating wire gages are too small in length to provide displacement information over long distances, exceeding typically many meters for example.
The invention further includes the instrument itself and the method of using the system.